Necking metal containers prior to end-piece connection has become widespread, particularly in the beverage industry. By reducing the diameter at an end of a container the amount of end piece material can be decreased to lower packaging costs, and containers can be stacked more readily (i.e., the reduced top of one container can fit within the larger diameter bottom of another), to accommodate storage, handling and display.
Numerous well-known necking techniques have been developed. Such techniques generally entail the use of external dies and/or rollers which act upon the outside of a container body. As used herein, a "die-necking" operation is one operation wherein a cylindrical container body and inward reducing die are axially aligned and opposingly advanced to force an open end of the container body through the reducing die. Due to the high compressive forces imparted to container bodies in die-necking operations, only a relatively small reduction in diameter per operation can be achieved without sidewall buckling or crumpling. As such, several successive die-necking operations are often necessary to achieve a desired reduced diameter.
In necking processes utilizing external rollers, one or more rollers contact the sidewall of a rotating container body near an open end thereof and are driven radially inward. A cylindrical member is internally and rotatably disposed at the open end of the container body to hold the open end during such processes. In most known processes, no internal support is provided in opposing relation to the inward progression of a forming roller, thereby resulting in process control problems which, in practice, limit the degree of inward necking. Further, in such known roll-forming processes, the configuration and relative positioning of the external roller and interfacing external/internal holders cause the open-end of the container body to be drawn through an extremely sharp radius therebetween (i.e., approaching a 90.degree. bend) to form a flange and generate a risk that metal slivers will be created within the container body. Such contemporaneous flange forming and production risk also limit, in practice, the degree of realizable inward necking.
Recently, a novel necking technique, known as "spin-flow necking" and described in U.S. Pat. Nos. 4,563,887 and 4,781,047, has been developed in which two internal members are provided to support and thereby control a rotating container body as an opposing external roller progresses inwardly and axially to neck the container, thereby allowing for a significant increase to the degree of inward necking that, in practice, can be realized in a single process step. More particularly, a first internal support member is configured and disposed in the open end of a container body in opposing relation to a radially driven external roller that is configured and disposed such that an angled first face thereof cams radially inward and axially towards the open end against a complimentarily angled face of the first support member to support and controllably reduce the container body diameter therebetween. Further, a second internal support member is configured and disposed adjacent to and outward from the first support member and relative to the configured external roller to rotatably hold the container body, and more importantly, such that a second angled face of the external roller cooperates with a complimentarily angled face of the second internal member during necking operations to support and thereby control the container body. The external roller is spring-loaded for measured axial movement towards the open end of the container as it cams against the first support member, and the second support member is spring-loaded for measured axial movement away from the first support member in response to the radial and axial movement of the external forming roller.
While spin-flow necking can effectively reduce the number of process steps necessary to achieve a desired neck diameter, and thereby reduce overall equipment requirements and production costs, it is sensitive to plug diameter variations. That is, it has been found that variations in the plug diameters of container bodies produced by different container body makers can, if not properly addressed, cause production problems during spin-flow necking (e.g., container body buckling, crumpling and wrinkling), and otherwise cause undesirable container variability upon completion of spin-flow necking (e.g., variations in container body height and variations in the configuration of the open-end edges of container bodies). In the later regard, and as will be appreciated by those skilled in the art, such container variability can present an impediment to the reduction of end-piece metal requirements.